Sib-size, birth order and risk of overweight in junior high school students in Japan: results of the Toyama Birth Cohort Study

OBJECTIVE: To investigate whether there is a relationship between sib-size, birth order and the risk of overweight in junior high school students. METHOD: Sib-size, birth order, height and weight of subjects and their parents, and other lifestyle factors were obtained through a questionnaire survey conducted between June and July 2002 from 7,959 junior high students in Toyama prefecture, Japan. Logistic regression analysis was performed to examine the associations. Boys and girls were analyzed separately. RESULTS: Boys from three-child families showed a significantly lower risk of overweight than only boys (odds ratio (OR)=0.66, p=0.010), and girls from larger families have a significantly lower risk than only girls (p for trend=0.001). Compared to middle-born girls, only girls have a significantly higher risk of overweight (OR=2.22, p<0.001). The risk of overweight in boys was significantly lower with increasing number of elder sibling or sister. However, increasing of any one sibling almost had the same risk reduction effect on the risk of overweight in girls (OR ranged from 0.74 to 0.76, p<=0.001). CONCLUSION: Children without siblings are the ones most at risk for overweight, especially in girls. Birth order and sib-size are possible risk factors for the development of overweight and obesity.